1. Field of the Invention
This invention relates to a process for the production of molded products composed of transparent resin having highly cured cross-linked structure and therefore having superior heat resistance as well as superior hardness. More particularly, the invention relates to a of from process for the production of molded products composed of transparent resin as mentioned above, which comprises incorporating a ring opening catalyst in an initial composition containing at least one glycidyl ester of containing at least one glycidyl ester and at least one other monomer to be polymerized at a low temperature, and polymerizing the initial monomer composition in a mold by irradiating the initial composition with an ionizing radiation at a low temperature to provide a polymer or copolymer in the form of plate, pipe or the like, and molding the resulting polymer or copolymer into a desired shape by means of compression molding, transfer molding or the like, and then curing the molded polymer or copolymer through the introduction of cross-linking by heating.
2. Description of the Prior Art
Up to this time, various methods have been tried to improve heat resistance of polymers by the introduction of cross-linking.. One example is a process in which polyethylene is irradiated with a radiation to introduce cross-linking therein. Another is a process for preparing a transparent resin by polymerizing methyl methacrylate, in which process ethylene dimethacrylate is used as a cross-linking agent in said methyl methacrylate to be polymerized so as to provide a cross-linked polymer having improved heat resistance. Instead of said ethylene dimethacrylate, various other polyfunctional monomers of similar type can also be used for the same purpose.
In general, a process for obtaining a polymeric molded product by two step molding method comprises the following. That is, the first step comprises polymerizing a monomer to be polymerized in a mold to provide a polymer in th form of plate, pipe or the like. The second step comprises remolding the resulting polymer into a desired shape. Therefore, in order to achieve the above propose, the polymer obtained in the first step must have thermoplasticity so as to enable the polymer obtained in the first step to be molded into the desired shape.
However, when polymerizing and cross-linking of a monomer to be polymerized are effected at the same time, the resulting polymer is hard and fragile due to the introduced cross-linked structure. In other words, the resulting polymer having cross-linkages by the glycidyl group cannot be molded into a desired shape, because it has losting its thermoplasticity.
Another known process is that in which glycidyl methacrylate is copolymerized with styrene, methyl methacrylate or the like followed by making a solution of the vinyl polymer thus obtained, to which phosphoric acid, amine or the like is added as a catalyst to bring about ring opening and cross-linking of glycidyl groups in subsequent polymerization ("High Polymer Chemistry", 17187 (1960); "J. Applied Polymer Sci", 5 58 (1961), etc.).
The other known prior art process is that in which glycidyl methacrylate polymers are cross-linked in the presence of a cyclic ether such as propylene oxide, tetrahydrofuran and the like, ring opening-graft polymerization of said cyclic ethers on said glycidyl methacrylate polymer being carried out.
However, none of such conventional curing processes is satisfactory to provide an advantageous process for the production of highly cured transparent resin molded products by the introduction of cross-linking. The reason is that, in order to uniformly disperse a cross-linking-ring opening catalyst into the polymer, in such prior art curing processes, polymers to be cured must be made into a solution or a finely dispersed emulsion thereof before they are cured by the introduction of cross-links. A transparent resin in the solid state cannot be obtained from the solution or emulsion of polymer, or the finely divided polymer, because on polymerizing the same, foam or the finely divided polymers are always present in the resulting molded polymer. Therefore, in the prior art there has been no advantageous method for obtaining thermoplastic polymers of glycidyl methacrylate or the like which are cured by the subsequent heat treatment.
In U.S.A. Pat. No. 2,580,901 dated Jan. 1, 1952, a copolymer of glycidyl acrylate and methacrylate and other monomer is described. However, the Patent does not disclose a process for producing a molded product composed of transparent resin having highly cured cross-linked structure which comprises vinyl-polymerizing the glycidyl ester and other monomer by means of an ionizing radiation so as not to cause cross-linking of the resulting polymer, and molding the vinyl polymer thus obtained into the desired shape, and heating the molded product to cause the cross-linking of the polymer.
In general, molded products formed from transparent resin having uniform optical properties have been prepared by the process comprising the first step polymerizing monomers to be polymerized in a mold to form a sheet or tube of polymer and the second step compression-molding or transfer-molding the sheet or tube thus obtained into a desired shape. Molded polymer having high strength and very high molecular weight has also been prepared by said process. When molded products composed the cross-linked polymer are to be prepared according to such process, a ring-opening and cross-linking catalyst must be incorporated in the initial monomer composition to be polymerized before polymerization of the monomers is carried out. If the catalyst is added after polymerizations, satisfactorily uniform dispersion thereof cannot be obtained, because the monomer has been polymerized at a conversion ratio of 90% or more after polymerizing the monomer in the mold, namely after cast molding of the monomer. It should be noted, however, that if such cross-linking catalyst is incorporated in initial monomer compositions before cast molding is carried out according to the conventional process in which a conventional radical inititator is added to said initial monomer compositions for initiating vinyl polymerization, the cross-linking catalyst will also be activated on heating the polymerization system to polymerizing temperatures, whereby the activated catalyst results in effecting the cross-linking reaction in the resulting polymer. Because the cross-linked polymer no longer has the thermoplasticity, it is impossible to mold the cross-linked polymer into end product having a desired shape.